The passage of high-speed trains necessitates very accurate track geometry. Track geometry has a direct impact on rail traffic. In fact, if the parameters that define the track are outside their nominal range train speeds must be reduced for reasons of safety and comfort, which can disturb all the traffic.
Monitoring rail track geometry for maintenance purposes thus proves indispensable to guaranteeing an optimum train speed. Any improvement in track geometry monitoring makes it possible to improve the management and maintenance of traffic on the rail network. Monitoring track geometry notably includes determining the signal of long base level and the long base straightness. Geometry monitoring of this kind proves particularly important in areas of fast change and areas of transition between ballast and a metal structure.
A known example of monitoring rail track geometry is employed on the French rail network using the IRIS320 high-speed train. This train travels over each of the high-speed lines every two weeks. This train carries out optical and inertial measurement of the track geometry on each of its passes.
The use of a train of this kind has a number of drawbacks. On the one hand, some track geometry parameters can change at a much higher frequency than that of the passage of the train. Thus the thermal expansion of a mixed steel-concrete structure, linked to temperature variations, notably induces relatively rapid track geometry variations. If this train measures a geometry variation that is out of range caused by a temporary factor, the speed of high-speed trains will have to be reduced although the track geometry is potentially satisfactory at the time of passage of high-speed trains operated commercially. As the passage of the IRIS320 train must interfere minimally with the operation of commercial trains, geometry monitoring during its passage is not necessarily representative of the geometry of the track during the passage of commercial trains. On the other hand, track geometry monitoring is not necessarily available in real time.
It is difficult to envisage significantly increasing the frequency of passages of monitoring trains of this kind given the cost of a geometry monitoring train of this kind and the impact that it can have on traffic.